1. Field of the Invention
The present invention generally relates to a plural-cell gas sensor or rather a multi-cell gas sensor for measuring concentration of a specific gas component such as oxygen, NOx, CO, HC and NH3 contained, for instance, in an exhaust gas exhausted from an automobile combustion engine and/or for controlling fuel-combustion in the engine. The present invention particularly relates to a plural-cell gas sensor including plural electrochemical cells including an oxygen pump cell and an oxygen monitor cell each comprising a solid electrolyte ceramic material; an internal space formed between the cells; and a heater for heating and activating the cells. More specifically, the present invention relates to an improved plural-cell gas sensor including at least an oxygen pump cell comprising a solid electrolyte ceramic layer; an internal space formed between the cells; and a heater substrate placed near the oxygen pump cell; the oxygen pump cell, being heated with the heater substrate and pumping out oxygen from the internal space, exhibiting improved activation.
2. Description of the Related Art
A conventional gas sensor including a plurality of solid electrolyte cells and a heater for heating the cells is disclosed, for instance, in U.S. Pat. Nos. 6,071,393, 6,344,134 and U.S. Patent Application Publication No. US-2004-0084309-A1 in which the heater is arranged in the vicinity of the gas sensor so as to activate the cells.
Conventionally, the plural-cell gas sensor includes at least one internal space formed by an oxygen pump cell and an oxygen monitor cell or an EMF cell (i.e., an electromotive force cell). An exhaust gas under measurement is introduced into the internal space through a gas-diffusion controlling aperture or inlet. The oxygen monitor cell partly constituting a wall of the internal space monitors an oxygen partial pressure in the internal space. The oxygen contained in the gas under measurement contacting an internal electrode of the oxygen pump cell that partly constitutes a wall of the internal space, dissociates into oxygen ions under a voltage applied across electrodes of the oxygen pump cell. The oxygen ions dissociated at the internal electrode of the pump cell flow through an oxygen-ion conductive solid electrolyte layer of the pump cell to an external electrode thereof. At the external electrode, the oxygen ions recombine to form oxygen that drains out to the ambient atmosphere or to the exhaust gas outside the sensor. The applied voltage is normally feedback-controlled by a monitor cell which monitors an EMF (electromotive force) corresponding to an oxygen partial pressure (or oxygen concentration) of the internal space so that the oxygen partial pressure inside the internal space is maintained at a constant low level value that is near zero. The oxygen ion current which has passed through the internal and external electrodes of the oxygen pump cell represents an oxygen concentration of the exhaust gas under measurement entering the internal space through the gas-diffusion limiting inlet. This is because the internal oxygen partial pressure is controlled to a constant low level. A residual gas in the internal space having a reduced oxygen partial pressure under an oxygen-pumping action of the pump cell may be admitted to a second internal space, wherein a specific gaseous component such as NOx, HC and CO in the residual gas is detected or analyzed by a gas detection cell partly constituting a wall of the second internal space.
Conventionally in this type of plural-cell gas sensor having an internal space formed therein, a heater has been placed in the vicinity of the sensor cells each comprising a solid electrolyte ceramic such as zirconia that becomes oxygen-ion conductive at high temperatures beyond about 300° C. However, because the oxygen pump cell is required to quickly and smoothly pump oxygen out of the internal space to the ambient gas atmosphere, an external air gap or channel for draining oxygen from the exterior electrode of the pump cell so as to vent to the ambient gas atmosphere has been formed between the external electrode of the pump cell and the heater substrate. In other words, in conventional sensors, a heater substrate has not been directly adhered to a surface of the exterior electrode of the pump cell, or otherwise. Rather, the heater substrate has been provided on a side of another, such as a monitor cell, that is not required to quickly pump out a large amount of oxygen from the internal space to the ambient atmosphere.
3. Problems to be Solved by the Invention:
However, in the type of plural-cell gas sensor that has an external air gap or channel formed between the oxygen pump cell and the heater so as to drain oxygen from the internal space to the external air gap, a heating efficiency or activating efficiency of the pump cell by the heater is not satisfactorily high enough to meet gas sensor requirements of late requiring fast activation of the sensor cells. This is because heat from the heater for activating the pump cell is thermally transferred via the external air gap formed between the pump cell and the heater substrate or via a spacer inserted between the heater substrate and a periphery of the pump cell for forming the external air gap.
In addition to such heating inefficiency caused by the heater substrate heating the oxygen pump cell, due to the external air gap formed between the pump cell external electrode and the heater substrate, the mechanical strength of the plural-cell gas sensor is comprised. Further, in co-firing a green laminate of plural cell layers laminated with a green heater substrate thereon, a careful firing process is needed to guarantee that a uniform external air gap is formed between the pump cell and the heater substrate.
In a modified conventional plural-cell gas sensor as disclosed in U.S. Patent Application Publication No. US-2004-0084309-A1, in which the heater substrate is not laminated on the side of the oxygen pump cell but rather on the side of an oxygen monitor cell, the activation of the oxygen pump cell is not satisfactory. This is because the heat from the heater substrate to the oxygen pump cell is transferred via the other cell layer, a spacer forming an internal space and via the internal space.
In addition, leakage current from the heater, superimposed on weak signals of the oxygen monitor cell or of the gas detection cell, may disadvantageously cause inaccurate gas measurements when the heater substrate is laminated on the side of the oxygen monitor cell or the gas detection cell. However, in that case, the external air gap to be formed between the oxygen pump cell and the heater substrate, which structurally weakens the plural-cell gas sensor, may be eliminated.